1. Field of the Invention
The invention relates to output circuits that provide output signals usable to control output devices.
2. Description of the Related Art
Output circuits are used for controlling output devices. In industrial applications, for example, output circuits are used to control devices such as fans, actuators, temperature control systems, lighting systems, and so on. One type of output circuit is an analog voltage output circuit. Typically, analog voltage output circuits provide an output voltage having a magnitude which is indicative of a desired output state of an output device. For example, in some applications, industry standards have been developed which specify that such voltage output circuits provide an output voltage in the range of 0 to 10 volts, with the output voltage having a magnitude that is proportional to a desired output condition. Thus, for a variable speed motor, an output circuit may provide an output voltage having a magnitude of 5 volts to cause the motor to operate at 50% maximum speed. An output device that is controlled in this manner is often referred to as a voltage-controlled output device.
Another type of output circuit is an analog current output circuit. Typically, analog current output circuits provide an output current having a magnitude which is indicative of a desired output state of an output device. For example, in some applications, industry standards have been developed which specify that such output circuits produce an output current in the range of 4 to 20 milliamps, with the output voltage or output current having a magnitude that is proportional to a desired output condition. For example, for a lighting system, an output circuit may provide an output current having a magnitude of 16 milliamps to cause the lighting system to operate at 75% maximum brightness. An output device that is controlled in this manner is often referred to as a current-controlled output device.
In general, an output device is either a voltage controlled output device or a current controlled output device, but not both. Generally, it is necessary that the output circuit be matched with the type of output device used, that is, that analog voltage output circuits be used with voltage-controlled output devices and analog current output circuits be used with current-controlled output devices.
When installing a new control system or modifying an existing control system, it is not always known which type of output devices will be or have been used. For example, when modifying an existing control system, where a new controller is installed but the output devices of the original system remain in place, it is generally not known in advance whether particular output devices are current controlled output devices or a-voltage controlled output devices. While this information can be determined by examining product specifications for the output device and/or by performing suitable measurements, this process is time consuming and not always possible or practical to perform.
According to a first aspect, a configurable analog output circuit comprises a current output circuit, a voltage output circuit, and a switch circuit. The current output circuit is capable of producing a current output signal usable to control a current through an output device. The voltage output circuit is capable of producing a voltage output signal usable to control a voltage across the output device. The switch circuit is capable of switching the analog output circuit between a first mode of operation in which the current output circuit is active and a second mode of operation in which the voltage output circuit is active. The switch circuit switches the analog output circuit between the first and second modes of operation responsive to a detected characteristic of the output device.
According to another aspect, a configurable analog output circuit comprises an output stage and a detection circuit. The output stage has first and second modes of operation. In the first mode of operation, the output stage provides an output device with an output current in the first mode of operation. In the second mode of operation, the output stage provides the output device with an output voltage. The detection circuit is coupled to the output stage. The detection circuit detects a characteristic of the output device and, in response, places the output stage in the first or second mode of operation.
According to another preferred aspect, a method of configuring an output circuit comprises simultaneously attempting to provide an output current and an output voltage to an output device. This step is performed by the output circuit in accordance with an input signal received by the output circuit. The method further comprises detecting (1) that providing the output device with the output current causes a voltage-providing capacity of the output circuit to be exceeded, or detecting (2) that providing the output device with the output voltage causes a current-providing capacity of the output circuit to be exceeded. In response to the detecting step (1), the output circuit is placed in a voltage output mode of operation in which the output circuit varies the output voltage in accordance with the input signal and provides the output voltage to the output device. In response to the detecting step (2), the output circuit is placed in a current output mode of operation in which the output circuit varies the output current in accordance with the input signal and provides the output current to the output device.